Turn on Nullable

[petrsvihlik]

https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references#nullable-contexts

Make code safer by turning on <Nullable>enable</Nullable> in .csproj and solving the warnings.

use if (obj is null) and if (obj is { }) for null checks
https://youtu.be/TJiLhRPgyq4?list=PLReL099Y5nRd04p81Q7p5TtyjCrj9tz1t&t=683

[petrsvihlik]

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator

[petrsvihlik]

switch to the latest version of c# via directory.props (https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version#configure-multiple-projects)

and unify all null checks (== null, != null, is null, is { }) to is null and is not null (logical patterns)